Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem–atmosphere CO2 exchange

Abstract

The stable isotope content of atmospheric CO₂ provides information about ecosystem carbon–water relations and biosphere–atmosphere carbon exchange. Virtually every isotope study within these fields has required air sample collection at remote locations followed by isotope analysis at a laboratory. This requirement severely limits sampling frequency and experiment duration. In this paper, we evaluate a tunable diode laser absorption spectrometer (TDL) for measuring the carbon isotope content of CO₂ at atmospheric mole fractions (350–700 μmol mol⁻¹) and isotopic abundance ($\text{δ}{}^{13}\text{C}$ of −6 to −16‰). Using infrared absorption, the TDL system determines the mole fractions of ${}^{12}\text{CO}{}_2$ and ${}^{13}\text{CO}{}_2$ independently, rather than their ratio as in mass spectrometry (MS). The ability of the instrument to measure isotope ratios ($\text{δ}{}^{13}\text{C}$) was tested outdoors in a grassland and compared to standard laboratory-based MS measurements made on field-collected flask samples. The TDL was operated at a sampling flow rate of 230 ml min⁻¹ and a sampling interval of 2 min for two intake heights. There was a consistent offset for $\text{δ}{}^{13}\text{C}$ of 1.77‰ between the TDL and MS measurements, and the standard deviation of the error (MS−TDL) was 0.35‰ (n=82). Removal of two outliers improved this standard deviation to 0.25‰ (n=80). After removing the offset, 62 out of 82 samples had absolute differences less than 0.3‰. Subsequent laboratory experiments indicated that the TDL/MS offset was caused by pressure broadening, and can be avoided in the future by calibrating the TDL with CO₂ mixed with air rather than nitrogen. Based on these results we estimate the precision for $\text{δ}{}^{13}\text{C}$ to be 0.25‰ for our sampling scheme. A similar comparison with flask-based measurements of CO₂ mole fraction (${}^{12}\text{CO}{}_2\text{+}{}^{13}\text{CO}{}_2$) made with a calibrated infrared gas analyzer indicated a TDL precision of 0.4% (1.6 μmol mol⁻¹ at 400 μmol mol⁻¹). The TDL was used to investigate the vertical and temporal variation in the carbon isotope content of respired CO₂ ($\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$) from the grassland. Measurements of $\text{δ}{}^{13}\text{C}$ of CO₂ in air were made during four separate nights at 1 and 60 cm height above ground. $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ did not vary with height, but it did vary from one night to the next. Hourly measurements of $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ showed it changed as much as 6.4‰ (−29.1±0.4 to −22.7±0.8‰) in a single night. Temporal changes in $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ during the night have not been reported in prior studies. Such observations could provide a new way to investigate temporal dynamics of the carbon substrates utilized for ecosystem respiration.